The present invention relates to electrically operated flow control valves of the type having a pressure balanced spool moveable in a bore for controlling flow from an inlet to an outlet ported along the bore. Such valves are employed where it is desired to provide a regulated flow to the outlet port in the face of a widely varying inlet pressure to the valve. Such requirements are typical of those found in automotive power steering applications where it is desired to electrically control the flow of hydraulic fluid to the power steering gear from an engine driven pump, the output of which varies with the engine speed.
Heretofore, automotive power steering systems employing electrically operated valves for controlling fluid flow to the steering gear have utilized electromagnetic operators such as a solenoid coil with an armature attached to a pressure balanced spool for controlling flow to outlet ports in a bore in which the spool is closely interfitted. In such electrically operated pressure balanced spool valves, it has been found that as the solenoid control signal current is increased to electromagnetically move the armature and spool to progressively close the flow from the inlet to the outlet ports in the bore, the increase in flow velocity through the outlet ports as the ports are progressively closed causes a significant pressure reduction at the inlet end of the spool and an accompanying force imbalance on the ends of the spool. Thus, the spool is accelerated toward the closed position by the pressure force imbalance causing a rapid fluctuation in the flow through the valve for a given electrical control signal to the solenoid. This rapid closing of the valve due to the dynamic pressure drop in the flow through the outlet ports has rendered the known valves difficult to utilize in certain applications where a relatively constant regulated flow is required, such as automotive power steering applications and has resulted in undesired response of the controlled system at certain valve positions.
Referring to FIG. 5 which is illustrative of measured performance of a prior art valve, it will be seen that at relative low back-pressures, i.e., 50 psig at the load, the flow in the prior art valves is quite erratic over the range of fluid supply pump speeds utilized. It will be further seen from FIG. 5, that at higher back pressures the flow in the prior art valves is somewhat stabilized at the higher pump speeds, but undergoes drastic excursions at the lower pump speeds.
With reference to FIG. 5, the sudden change in flow rate through the valve for various levels of back-pressure due to the load in the system as experienced by the prior art valve is shown for the situation of the valve inlet connected to a variable speed pump supply; and, the effect is dramatically evident in the pump shaft speed range of 2,000 to 5000 rpm.
Referring to FIG. 6, which is also illustrative of measured performance of a prior art valve, it will be seen that even with no current (0 milliaperes) to the valve coil of a prior art device and with the valve in the fully open condition, at certain pump speeds (supply pressures), the flow is substantially reduced by pressure imbalance on the spool. Referring to FIG. 6, it will be further seen that, at various levels of valve coil energization, the prior art valve produces flow which is quite erratic at low and high supply pump speeds.
Accordingly, it has been desired to find a way or means of electrically controlling flow through a spool valve in response to widely varying inlet pressure conditions, particularly those resulting from a variable speed pump supply to the valve inlet and to provide a regulated flow in a manner that results in a valve having relatively low manufacturing costs and reliable operation of the valve.